It is well known in the art relating to the manufacture of lithographic printing plates to utilize a photopolymerizable composition as the image-forming layer. Typical photopolymerizable compositions for this purpose comprise one or more addition-polymerizable ethylenically-unsaturated compounds and a photopolymerization initiator system. The addition-polymerizable compounds can be monomers, oligomers or polymers, or mixtures thereof, and frequently the photopolymerizable composition will also include a film-forming polymeric binder.
Photopolymerizable compositions of the type which are suitable for direct digital exposure are of increasing importance to the printing industry. Such compositions find particular utility in so-called "direct-to-plate" or "computer-to-plate" systems which utilize image setters. Practical considerations have resulted in visible-light-emitting lasers, such as low power argon-ion lasers, being preferred for use with plate-capable image setters. The need for high writing speeds, coupled with the constraint of the low-powered lasers favored in the industry, has resulted in a requirement for printing plates that have very high photosensitivities. However, high photosensitivity alone is not enough as it is essential that the photopolymerizable composition also have good shelf-life.
In an effort to meet the dual objectives of high photosensitivity and good shelf-life, the prior art has proposed the use of a wide variety of different photopolymerization initiator systems, but has not been fully successful in simultaneously meeting the dual objectives. Among the most effective prior art photopolymerization initiator systems are those which comprise a spectral sensitizing dye and an N-aryl-.alpha.-amino acid, such as N-phenylglycine (NPG) or a derivative thereof, as a co-initiator. Thus, for example, U.S. Pat. No. 4,278,751 describes an initiator system comprising NPG and an amine-substituted ketocoumarin; U.S. Pat. Nos. 4,868,092 and 4,965,171 describe an initiator system in which the spectral sensitizing dye is a xanthene, coumarin or merocyanine and which includes both NPG, or certain derivatives thereof, and a diaryliodonium salt; and U.S. Pat. No. 5,153,236 describes an initiator system which includes an N-aryl-.alpha.-amino acid, such as NPG, and in which the spectral sensitizing dye is a thioxanthone, an isoalloxanine or a coumarin. Photopolymerizable compositions containing an initiator system comprising a coumarin, an iodonium salt and NPG are also described in J. P. Fouassier et al, "A New Three-Component System In Visible Laser Light Photo-Induced Polymerization", Journal Of Imaging Science And Technology, Vol. 37, No. 2, March/April, 1993.
In addition to the use of N-aryl-.alpha.-amino acids as co-initiators, it is also known to employ O-aryl and S-aryl compounds as described for example, in U.S. Pat. No. 4,939,069, issued Jul. 3, 1990.
Photoinitiating systems of the bimolecular type consisting of a radical generator and a sensitizing dye and mechanisms whereby they are believed to function are described in Yamaoka et al, "N-Phenylglycine-(Thio)xanthene Dye Photoinitiating System and Application to Photopolymer for Visible Laser Exposure", Journal of Applied Polymer Science, Vol. 38, 1271-1285 (1989). As pointed out by Yamaoka et al, bimolecular type photoinitiators consisting of N-phenylglycine and a xanthene or thioxanthene dye exhibit high initiating efficiency. However, such systems are deficient in regard to shelf-life.
Fluorone dyes are shown to work well as visible light photoinitiators in combination with NPG and iodonium salts in an article by Thomas L. Marino et al, entitled "Chemistry And Properties Of Novel Fluorone Visible Light Photoinitiators" published in the Proceedings of the Radtech '94 Conference, Vol. 1 page 1691, 1994 by Radtech International North America. The fluorone dyes described provide panchromatic absorption throughout the visible spectrum.
Use of NPG as a co-initiator in a photopolymerization initiator system provides excellent photospeed, but the composition tends to rapidly lose photospeed on aging, i.e., its shelf-life is poor. Thus, for example, D. F. Eaton has noted that the improvements in photosensitivity provided by amine activators in general and by NPG in particular are lost within days of preparation of photosensitive films containing such materials. See "Dye Sensitized Photopolymerization" by D. F. Eaton in Advances In Photochemistry, Vol. 13, D. Volman, K. Gollnick and G. S. Hammond, eds., Wiley Interscience (1986).
Arimatsu, U.S. Pat. No. 5,378,579, describes a photopolymerizable composition in which the NPG co-initiator has been derivatized with a compound such as glycidyl methacrylate in an effort to improve shelf-life. Such use of glycidyl methacrylate is, however, highly undesirable from an environmental standpoint, because of concerns relating to toxicity, and improvement in shelf-life is marginal at best.
Additional references relating to the use of iodonium salts in photopolymerizable compositions include Japanese Patent No. 1,660,964 (Patent Application No. 58-198085, filed Oct. 21, 1983, KOKAI No. 60-88005, published May 17, 1985, KOKOKU No. 3-62162, published Sep. 25, 1991) which describes a photopolymerization initiator system comprising a 3-ketocoumarin and a diaryl iodonium salt; U.S. Pat. No. 4,921,827, issued May 1, 1990, which describes a photopolymerization initiator system comprising at least one merocyanine sensitizer containing a constrained alkylamino group and an initiator selected from the group consisting of photosensitive iodonium salts, sulfonium salts and halomethyl s-triazines; and European Patent Application No. 0 290 133 which describes a photopolymerization initiator system comprising an aryliodonium salt, a sensitizer and an electron donor having an oxidation potential that is greater than zero and less than or equal to that of p-dimethoxybenzene.
While the photopolymerizable compositions of the prior art meet many of the needs of the lithographic printing plate industry, they still do not fully satisfy the dual requirements of high photospeed and good shelf-life that are urgently needed. It is toward the objective of better meeting these dual requirements that the present invention is directed.